In situ method of determining the thrust on valve components

Abstract

Without disassembling a valve stem from a valve, the valve is decoupled from a valve actuator such that the valve stem and attached valve disc or body may shift to and from a seated position without interference by the actuator. Known loads are applied to the decoupled valve stem to cause a strain gauge output which corresponds to the known load. The strain gauge is mounted to the valve stem or other force reactive components of the valve which react by changing in dimension in response to the applied known load. The relationship of the applied load to the strain gauge output is determined to thereby calibrate the valve stem. The load is removed and the valve stem is recoupled to the valve actuator for operation. From the strain gauge output during either or both static and dynamic operation of the valve, the thrust on the valve stem is directly obtained and is used to evaluate the performance of the valve and actuator.

I claim: 
     
       1. A method of determining the thrust exerted on the force reactive portions of a valve during movement of a valve body, the valve body being movable to and from a closed position in which the valve body is seated in an associated valve seat and closes a flow path between an inlet and an outlet to the valve, the valve body being coupled to a valve stem and moved to and from the closed position in response to a valve actuator which applied an axial valve actuating force to a portion of the valve stem which is spaced from the valve body, the method comprising the steps of: positioning a strain gauge on a force reactive portion of the valve, the strain gauge providing an output corresponding to dimensional variations of the force reactive portion of the valve at the location of the strain gauge in response to application of the valve actuating force to the valve stem;   decoupling the valve stem from the valve actuator;   calibrating the strain gauge by applying at least one known test load to the decoupled valve stem without removing the valve stem from the valve and monitoring the output from the strain gauge in response to the known test load;   removing the test load;   recoupling the valve steam to the valve actuator;   monitoring the output from the strain gauge in response to the application of a valve actuating force by the valve actuator.   
     
     
       2. A method according to claim 1 including the step of comparing the output from the strain gauge with a reference. 
     
     
       3. A method according to claim 2 including the step of detecting and indicating differences between the strain gauge output and the reference. 
     
     
       4. A method according to claim 2 including the step of adjusting the force applied by the valve actuator in response to the comparison. 
     
     
       5. A method according to claim 1 including the step of adjusting the force applied by the valve actuator 
     
     
       6. A method according to claim 1 including the step of comparing the strain output with a reference to determine drag on the valve stem during operation of the valve toward a seated position. 
     
     
       7. A method according to claim 6 also including the step of comparing the strain gauge output with a reference to determine drag on the valve stem during operation of the valve away from the seated position. 
     
     
       8. A method according to claim 1 including the step of comparing the strain gauge output with a reference to determine thrust exerted on the valve stem when the valve body is seated in the valve seat. 
     
     
       9. A method according to claim 8 including the step of adjusting the force applied by the valve actuator in response to the comparison. 
     
     
       10. A method according to claim 1 in which the positioning step comprises the step of positioning the strain gauge on a portion of the valve stem. 
     
     
       11. A method according to claim 1 in which the positioning step comprises the step of positioning the strain gauge on a yoke which couples the valve actuator to the valve stem. 
     
     
       12. A method according to claim 1 including the step of comparing the strain gauge output with a reference to determine drag on the valve stem during operation of the valve away from a seated position.